A method for producing planar extended electrodes with nanoscale spacings that exhibit very large SERS signals, with each nanoscale gap having one well-defined hot spot. The resulting highly sensitive substrate has extended metal electrodes separated by a nanoscale gap. The electrodes act as optical antennas to enhance dramatically the local electromagnetic field for purposes of spectroscopy or nonlinear optics. SERS response is consistent with a very small number of molecules in the hotspot, showing blinking and wandering of raman lines. Sensitivity is sufficiently high that SERS from physisorbed atmospheric contaminants may be detected after minutes of exposure to ambient conditions.
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32. A highly sensitive substrate comprising:
a pair of planar extended metal electrodes fabricated on a substrate, said pair of electrodes being separated by a nanoscale gap with an interelectrode separation on a scale of few nanometers;
wherein said nanoscale gaps exhibit very large surface-enhanced raman signals, with each nanoscale gap having one well-defined hot spot.
1. A method for producing a plurality of highly sensitive substrates, the method comprising the steps of:
lithographically defining a plurality of electrodes joined by one or several constrictions, each constriction being less than 500 nm wide;
depositing an electrode metal;
performing liftoff after metallization;
cleaning said constrictions in oxygen plasma to remove organic residue from the lithography process; and
performing electromigration of the constrictions until a desired interelectrode conductance is reached.
16. A method for producing a plurality of highly sensitive substrates comprising the steps of:
depositing an electrode metal;
lithographically defining a plurality of electrodes joined by one or several constrictions, each less than approximately 500 nm wide;
subtractive patterning of the electrode metal by etching; removal of remaining resist;
cleaning said constrictions in oxygen plasma to remove organic residue from the lithography process;
and performing electromigration of the constrictions until a desired interelectrode conductance is reached.
30. A method for performing surface enhanced raman spectroscopy on a molecule of interest, the method comprising the steps of:
lithographically defining a plurality of electrodes joined by several constrictions, each approximately 100 nm wide;
depositing an electrode metal;
performing liftoff after metallization;
cleaning said constrictions in oxygen plasma to remove organic residue from the lithography process;
performing electromigration of the constrictions until a desired interelectrode conductance is reached;
depositing a molecule of interest; and
performing raman characterization of said molecule with a raman microscope.
31. A method for performing simultaneous surface enhanced raman spectroscopy and electronic conduction measurements on a molecule of interest, the method comprising the steps of:
lithographically defining a plurality of electrodes joined by one or several constrictions, each approximately 100 nm wide;
depositing an electrode metal;
performing liftoff after metallization;
cleaning said constrictions in oxygen plasma to remove organic residue from the lithography process;
performing electromigration of the constrictions until a desired interelectrode conductance is reached;
depositing a molecule of interest; performing raman characterization of said molecule with a raman microscope; and
simultaneously using the electrodes to measure the electronic conduction across the interelectrode gap.
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The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/889,668 entitled “Lithographic Method For Producing Highly Sensitive Substrates For Surface-Enhanced Raman Spectroscopy” and filed on Feb. 13, 2007.
The above cross-referenced related application is hereby incorporated by reference herein in its entirety.
This invention was made with government support under National Science Foundation Grant No: DGE-0504425. The government has certain rights in the invention.
1. Field of the Invention
The present invention relates to a new process and device design for producing substrates for highly sensitive surface-enhanced Raman spectroscopy and multimodal sensing. The resulting devices are potentially very useful for chemical sensing for a variety of applications. The large surface enhancement of electromagnetic fields may also have uses in nonlinear optics and plasmonic signal generation or routing.
2. Brief Description of the Related Art
Multifunctional sensors with single-molecule sensitivity are greatly desired for a variety of sensing applications, from biochemical analysis to explosives detection. Chemical and electromagnetic interactions between molecules and metal substrates are used in surface-enhanced spectroscopies to approach single molecule detection. See M. Moskovits, Rev. Mod. Phys. 57, 783 (1985). Electromagnetic enhancement in nanostructured conductors results when incident light excites local electronic modes, producing large electric fields in a nanoscale region, known as a “hot spot”, that greatly exceed the strength of the incident field. This local field enhancement is the mechanism responsible for a variety of “surface-enhanced” spectroscopies, including surface-enhanced Raman (SERS), surface-enhanced infrared adsorption (SEIRA), and surface-enhanced fluorescence (SEF). Hot spots can lead to particularly large enhancements of Raman scattering, since the Raman scattering rate is proportional to |E(ω)|2|E(ω′)|2 at the location of the molecule, where E(ω) is the electric field component at the frequency of the incident radiation, and E(ω) is the component at the scattered frequency. Still, substrates that give large Raman enhancements are often useful for SEIRA and SEF as well. Large local field enhancements are also useful for nonlinear optical processes, and have been discussed in the context of optical information processing (Chang et al., Nature Physics 3, 807-812 (2007)).
It has been an ongoing challenge to design and fabricate a substrate for systematic surface-enhanced Raman spectroscopy (SERS) at the single molecule level. Achieving SERS with single-molecule sensitivity was first clearly demonstrated using random aggregates of colloidal nanoparticles. K. Kneipp, et al., “Colloidal silver rhodamine 6 g fluorescence spectroscopy gold,” Phys. Rev. Lett. 78, 1667 (1997); S. Nie, S. R. Emory, Science 275, 1102 (1997); H. Xu, E. J. Bjerneld, M. Käll, L. Börjesson, Phys. Rev. Lett. 83, 4357 (1999); and A. M. Michaels, J. Jiang, L. Brus, J. Phys. Chem. B 104, 11965 (2000). While numerous other metal substrate configurations have been used for SERS, including engineered nanoparticles made chemically, nanostructures defined by bottom-up patterning and traditional lithographic approaches, the most sensitive substrate geometries rely on closely adjacent subwavelength nanoparticles or nanostructures. See J. Jackson and N. J. Halas, Proc. Nat. Acad. Sci. U.S. 101, 17930-17935 (2004); H. Wang, C. S. Levin and N. J. Halas, J. Am. Chem. Soc. 127, 14992-14993 (2005); C. L. Haynes, R. P. van Duyne, J. Phys. Chem. B 105, 5599 (2001); L. Qin, et al., Proc. Nat. Acad. Sci. U.S. 103, 13300 (2006); D. P. Fromm, et al., J. Chem. Phys. 124, 061101 (2006).
In this geometry, incident light may excite the collective resonance of the pair of coupled nanostructures, resulting in large field enhancements within the interparticle gap. See A. J. Hallock, P. L. Redmond, and L. E. Brus, Proc. Nat. Acad. Sci. U.S. 102, 1280-1284 (2005); P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. Stockman, Nano Letters 4, 899-903 (2004). Fractal aggregates of nanoparticles can further increase field enhancements by focusing plasmon energy from larger length scales down to particular nanometer-scale hotspots. See Z. Wang, S. Pan, T. D. Krauss, H. Du, L. J. Rothberg, Proc. Nat. Acad. Sci. U.S. 100, 8638 (2003); K. Li, M. I. Stockman, D. J. Bergman, Phys. Rev. Lett. 92, 227402 (2003). However, precise and reproducible formation of such gaps and assemblies in predetermined locations has been extremely challenging. An alternative approach is tip-enhanced Raman spectroscopy (TERS), in which the incident light excites an interelectrode plasmon resonance localized between a sharp, metal scanned probe tip and an underlying metal substrate. See D. Richards, R. G. Milner, F. Huang, F. Festy, J. Raman Spectrosc. 34, 663 (2003); C. C. Neascu, J. Dreyer, N. Behr, M. B. Raschke, Phys. Rev. B 73, 193406 (2006). A similar approach was recently attempted using a mechanical break junction. See J.-H. Tian, et al., J. Am. Chem. Soc. 128, 14748 (2006). While useful for surface imaging, TERS requires feedback to maintain a few-nm tip-surface gap, and is not scalable or readily integrated with other sensing modalities.
In a preferred embodiment, the present is an arrangement of pairs of planar metal electrodes fabricated on a substrate; these electrodes are separated from each other by one or more nanoscale gaps, with interelectrode separations on the few-nanometer scale; these electrodes act as nanoscale optical antennas such that, under optical illumination, the local electromagnetic field in the interelectrode gap(s) is greatly enhanced compared to the incident field. In the preferred method of the present invention, lithography is used to produce constrictions between electrode pads and then electromigration is used to break those constrictions into nanoscale gaps between the electrode pads. Using the method of the present invention, highly sensitive substrates can be produced on a mass scale using conventional industrial photolithography and wafer-probing.
With the resulting highly sensitive nanogap substrates, the Raman response is great enough to see few or even single molecules and is so sensitive that one can see atmospheric contaminants. Further, the method of the present invention fabricates SERS “hotspots” routinely, in precise locations, ready for integration with other sensing modalities (e.g. conduction, lab-on-a-chip). Each nanoscale gap may produce one hotspot. SERS emission is localized to just the nanoscale gap, not metal edges or pads. The resulting substrates have many potential uses, and may be particularly useful in sensing applications or other nonlinear optics and plasmonic applications.
In a preferred embodiment, the present invention is a method for producing highly sensitive substrates for surface-enhanced Raman spectroscopy. The method comprises the steps of lithographically defining a plurality of electrodes joined by one or several constrictions, each constriction being less that approximately 500 nm wide; depositing an electrode metal; performing liftoff after metallization; cleaning the constrictions in oxygen plasma to remove organic residue from the lithography process; performing electromigration of the constrictions until a desired interelectrode conductance is reached. The method may further comprise depositing a molecule of interest and performing Raman characterization of said molecule with a Raman microscope. The step of lithographically defining a plurality of electrodes may comprise, for example, photolithography or e-beam lithography. An alternative embodiment would be subtractive patterning: coat the desired substrate with a metal film; perform lithography to define areas of metal to be removed by subsequent etching; etch the exposed areas of metal; strip off the remaining resist and remove organic residue with oxygen plasma; then electromigrate as above. In a preferred embodiment, each electrode is hundreds of microns on a side and each constriction is approximately 100 nm wide. The step of depositing an electrode metal may comprise deposition using e-beam evaporation of 15 nm of Au with a 1 nm Ti adhesion layer.
The step of electromigration of the constrictions may be performed using a computer-controlled voltage source and current meter, may be performed until a final resistance of approximately 100 kOhms divided by the number of constrictions in parallel is reached and/or may be performed to create nanometer-scale gaps in said constrictions. The step of electromigration of the constrictions may be automated at the batch level using a standard automated probe system common to the semiconductor industry. A substrate with an array of such constrictions may be used as a master for other fabrication techniques such as imprint lithography or microcontact printing. Depending on the details of reproduction method electromigration may not be required for copies so produced.
In the preferred embodiment, the step of performing Raman characterization is performed using a WiTek scanning Raman system. Other light gathering methods are possible, including integrated local lenses and optical fibers. The method may further comprise the step of wiring up the electrodes using a wire-bonder.
In another embodiment, the present invention is a highly sensitive substrate. The highly sensitive substrate comprises a pair of planar extended metal electrodes fabricated on a substrate, said pair of electrodes being separated by a nanoscale gap with an interelectrode separation on a scale of few nano-meters. The nanoscale gaps exhibit very large SERS signals, with each nanoscale gap having one well-defined hot spot. In another embodiment, the highly sensitive substrate comprises a plurality of the pairs of planar extended metal electrodes. The pair of planar extended metal electrodes may form a nanoscale optical antenna and under optical illumination an electromagnetic field in the nanoscale gap separating the pair of electrodes is greatly enhanced compared to an incident field.
Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a preferable embodiments and implementations. The present invention is also capable of other and different embodiments and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention.
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description and the accompanying drawings, in which:
The present invention demonstrates that planar, extended metal electrodes separated by a nanoscale gap act as optical antennas to enhance dramatically the local electromagnetic field for purposes of spectroscopy or nonlinear optics. The present invention also provides a scaleable and highly reliable method for producing planar extended electrodes with nanoscale spacings that exhibit very large SERS signals, with each nanoscale gap having one well-defined hot spot. Confocal scanning Raman microscopy demonstrates the localization of the enhanced Raman emission. SERS response is consistent with a very small number of molecules in the hotspot, showing blinking and wandering of Raman lines. Sensitivity is sufficiently high that SERS from physisorbed atmospheric contaminants may be detected after minutes of exposure to ambient conditions. The Raman enhancement for p-mercaptoaniline (pMA) is estimated from experimental data to exceed 109. Finite-difference time-domain (FDTD) modeling of realistic structures reveals a rich collection of interelectrode plasmon modes that can readily lead to SERS enhancements as large as 5×1010 over a broad range of illumination wavelengths. These structures hold the promise of integration of single molecule SERS with electronic transport measurements, as well as other near-field optical devices.
Structures in accordance with a preferred embodiment of the present invention are fabricated on a Si wafer topped by 200 nm of thermal oxide. Electron beam lithography is used to pattern “multi-bowtie” structures as shown in
The electrode pads 110 are connected by multiple constrictions 120, as shown. The constriction widths L1 in a preferred embodiment may be approximately 80-100 nm, readily within the reach of modern photolithography. This preferred width is not critical to the invention. Narrower widths would make the electromigration process easier. The length L2 (shown in
After evaporation of 1 nm Ti/15 nm Au followed by liftoff in acetone, the electrode sets are cleaned of organic residue by exposure to O2 plasma for 1 minute. The multi-bowties, which comprise a plurality of individual bowties such as shown in
Post-migration high resolution scanning electron microscopy (SEM) shows interelectrode gaps ranging from too small to resolve to several nanometers. There are no detectable nanoparticles in the gap region or along the electrode edges. Based on electromigration of 283 multibowties (1981 individual constrictions), 76.8% of multibowties have final resistances less than 100 k, and 43% have final resistances less than 25 kΩ. This yield, already high, can be improved with better process control, particularly of the lithography and liftoff. The resulting device geometries may be realizable through other fabrication methods, such as subtractive patterning, or masked/angled deposition, rather than additive patterning and electromigration as in the preferred embodiment.
In the demonstrated embodiment, the optical properties of the resulting nanogaps are characterized using a WITec CRM 200 scanning confocal Raman microscope in reflection mode, with normal illumination from a 785 nm diode laser. Using a 100× objective the resulting diffraction-limited spot is measured to be Gaussian with a full-width at half-maximum of 553 nm. The Au electrodes are clearly resolved. Rayleigh scattered light from these structures shows significant changes upon polarization rotation, while SERS response is approximately independent of polarization.
Freshly cleaned nanogaps show no Stokes-shifted Raman emission out to 3000 cm−1. However, in 65% of clean nanogaps examined, a broad continuum background is seen, decaying roughly linearly in wavenumber out to 1000 cm−1 before falling below detectability. This background is spatially localized to a diffraction-limited region around the interelectrode gap, and is entirely absent in unmigrated junctions. The origin of this continuum, similar to that seen in other strongly enhancing SERS substrates such as that disclosed in A. M. Michaels, J. Jiang, L. Brus, J. Phys. Chem. B 104, 11965 (2000), is likely inelastic electronic effects in the gold electrodes. See M. R. Beversluis, A. Bouhelier, L. Novotny, Phys. Rev. B 68, 115433 (2003). In samples coated with molecules, this background correlates strongly with visibility of SERS. Junctions without this background never show SERS signals. Like the SERS, this background is approximately polarization independent. Temporal fluctuations of this background in clean junctions are minimal, strongly implying that fluctuations of the electrode geometry are not responsible for SERS blinking.
The SERS enhancement of the junctions has been tested using various molecules. The bulk of testing employing para-mercaptoaniline (pMA), which self-assembles onto the Au electrodes via standard thiol chemistry. Particular modes of interest are carbon ring modes at 1077 cm−1 and 1590 cm−1. The emission is strongly localized to the position of the nanogap. No Raman signal is detectable either on the metal films or at the edges of the metal electrodes.
This blinking and wandering is seen routinely in these junctions. Such Raman response has been observed from several molecules, including self-assembled films of pMA, p-mercaptobenzoic acid (pMBA), a Co-containing transitionmetal complex (see J. Ciszek, et al., J. Am. Chem. Soc. 128, 3179 (2006)), and spin-coated poly(3-hexylthiophene) (P3HT). These molecules have distinct Raman responses and all show blinking and wandering in the junction hotspots.
Another indicator of very large enhancement factors in these structures is sensitivity to exogenous, physisorbed contamination. Carbon contamination has been discussed in the context of both SERS and TERS. D. Richards, R. G. Milner, F. Huang, F. Festy, J. Raman Spectrosc. 34, 663 (2003); A. Otto, J. Raman Spectrosc. 33, 593 (2002). This substrate is sensitive enough to examine the arrival of such contaminants. Clean nanospaced junctions with no deliberately attached molecules initially show no Raman features beyond the continuum background mentioned above. However, after exposure to ambient lab conditions for tens of minutes, SERS signatures in the sp2 carbon region between 1000 cm−1 and 1600 cm−1 are readily detected, localized to the interelectrode gap, like all SERS signatures on these substrates. Interestingly, nanojunctions that have been coated with a self-assembled monolayer (SAM) (for example, pMA) do not show this carbon signature, even after hours of ambient exposure. Presumably this has to do with the extremely localized field enhancement in these structures, with the SAM sterically preventing physisorbed contaminants from entering the region of enhanced near field.
Recently arrived contaminant SERS signatures disappear in tens of seconds at high laser intensities (26 kW/cm2), presumably due to desorption. SERS from covalently bound molecules is considerably more robust, persisting for hours for intensities below 10.4 kW/cm2. SEM examination of the nanogaps shows no obvious signs of optically induced damage or melting afterexposure to intensities that would significantly degrade nanoparticles. See P. Schuck, D. Fromm, A. Sundaramurthy, G. Kino, W. Moerner, Phys. Rev. Lett. 94, 017402 (2005). The large extended pads likely aid in the thermal sinking of the nanogap region to the substrate.
Estimating enhancement factors rigorously is notoriously difficult, particularly when the hotspot size is not known. Although SERS enhancement volume measurements are feasible using molecular rulers, such an approach is not feasible with such small nanogaps. See S. Lal, N. K. Grady, G. P. Goodrich and N. J. Halas, Nano Letters 6, 2338-2343 (2006). Fabrication of nanogaps directly on Raman-active substrates allows some estimation of the enhanced volume in the direction normal to the substrate. Junctions fabricated directly on Raman-active substrates (Si with no oxide; GaAs) show essentially no clearly detectable enhancement of the substrate Raman response in the gap region. This suggests that the region of electromagnetic enhancement is strongly confined to the thickness of the metal film electrodes.
FDTD calculations were used to understand the strong SERS response in these structures and estimate the enhancement factors theoretically. These calculations also allow an estimate of the hotspot volume, so that the data in
These calculations predict that there should be large SERS enhancements across a broad bandwidth of exciting wavelengths because of the complicated mode structure possible in the interelectrode gap. Nanometer-scale asperities from the electromigration process break the interelectrode symmetry of the structure. The result is that optical excitations at a variety of polarizations can excite many interelectrode modes besides the simple dipolar plasmon commonly considered. This broken symmetry also leads to much less dependence of the calculated intensity on polarization direction, as seen experimentally. The calculations confirm that the field enhancement is confined in the normal direction to the film thickness. Laterally, the field enhancement is confined to a region comparable to the radius of curvature of the asperity. These calculations predict a purely electromagnetic enhancement that can approach 1011, approaching that sufficient for single-molecule sensitivity.
Using the calculated effective hotspot area and data from the device in
The present invention provides a SERS substrate capable of extremely high sensitivity for trace chemical detection. Unlike previous substrates, these nanojunctions may be mass fabricated in controlled positions with high yield using a combination of standard lithography and electromigration. The resulting hotspot geometry is predicted to allow large SERS enhancements over a broad band of illuminating wavelengths. Other nonlinear optical effects should be observable in these structures as well. The extended electrode geometry and underlying gate electrode are ideal for integration with other sensing modalities such as electronic transport. Tuning molecule/electrode charge transfer via the gate electrode may also enable the direct examination of the fundamental nature of chemical enhancement in SERS.
Due to the large enhancements possible with the nanogaps, contamination from airborne absorbates occurs readily in the absence of assembled molecules on the nanogap surface. We have observed the absorption of contaminates onto the surface of clean nanogaps in as little as 10 minutes. Collecting Raman spectra every 4 seconds, we can observe the appearance of contaminants on the surface as seen in
The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.
Natelson, Douglas, Ward, Daniel Robert, Keane, Zachary Kyle
Patent | Priority | Assignee | Title |
8810787, | Dec 09 2008 | IMEC; KATHOLIEKE UNIVERSITEIT LEUVEN, K U LEUVEN R&D | Single molecule optical spectroscopy in solid-state nanopores in a transmission-based approach |
9329339, | Sep 02 2009 | Agency for Science, Technology and Research | Plasmonic detector and method for manufacturing the same |
Patent | Priority | Assignee | Title |
6614742, | Dec 14 1999 | Fuji Xerox, Ltd. | Optical head, magneto-optical head, disk apparatus and manufacturing method of optical head |
7857959, | Nov 19 2004 | The Trustees of Boston College | Methods of fabricating nanowires and electrodes having nanogaps |
20010009541, | |||
20020153874, | |||
20030112542, | |||
20030198146, | |||
20040211906, | |||
20050030993, | |||
20050157393, | |||
20070058686, |
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